Binaural hearing aid system



United States Patent 3,509,289 BINAURAL HEARING AID SYSTEM Robert J. Briskey and William G. Ely, Des Plaines, and Kenneth R. Wruk, Joliet, Ill., assignors to Zenith Radio Corporation, Chicago, [1]., a corporation of Delaware Filed Oct. 26, 1967, Ser. No. 678,289 Int. Cl. H04r 25/00, 5/00; G02c 11/06 US. Cl. 179107 8 Claims ABSTRACT OF THE DISCLOSURE Background of the invention The present invention relates in general to hearing aid systems, and more particularly, to binaural hearing aid systems with Automatic Gain Control.

In the human hearing system the amplitude of the sound received by the ipsilateral or closer ear is of greater amplitude than the sound received by the contralateral or opposite ear. This difference in sound level, although often minute, is very important in that it allows listeners to perceive the relative direction of an incident sound. Heretofore, when providing hearing aids for persons with binaural hearing loss this fact was largely ignored. As a result, users of binaural hearing aids often suffered severe loss of spatial separation and experienced great difficulty in orientating with respect to sources of sound.

The problem of spatial disorientation was especially prevalent in those binaural hearing aid systems incorporating automatic gain control (AGC). With AGC the gain of each audio amplifier channel was inversely proportional to the level of the input signal to that channel and, as a consequence, the difference in the acoustic output levels of the ipsilateral and contralateral channels was less than the difference in the input signal levels. Thus, a conventional binaural hearing aid system with AGC almost completely destroyed the interaural level differential which would otherwise exist as a result of head shadow and the directional characteristics of the human ear.

Summary of the invention It is a general object of the present invention therefore to provide an improved binaural hearing aid system which provides an improved sense of spatial separation.

It is a more specific object of the invention to provide an economical binaural heading aid system having automatic gain control and yet capable of maintaining interaural level differences between incident sounds.

It is a still more specific object of the invention to provide an improved binaural hearing aid system which enhances the interaural level differences of high-level in cident sounds.

To effect the objects of the invention there is provided a binaural hearing aid system of the type comprising a first amplifying channel for amplifying and reproducing a first incident sound and a second amplifying channel for amplifying and reproducing a second incident sound. Means comprising first and second automatic gain control circuits responsive to the output levels of the first and second amplifying channels are included for developing first and second independent control effects. Funther included are means for applying the first control effect to the second channel and the second control effect to the first channel to inversely vary the gain of one channel with variations in the output level of the other. Crosscoupling means connected between the first and second automatic gain control circuits are included for causing the relative level difference of the output signals from the amplifying channels to be related in a predetermined manner to the relative level difference of the first and second incident sounds.

Brief description of the drawings The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

FIGURE 1 is a block diagram of a binaural hearing aid system embodying the present invention; and

FIGURE 2 is a schematic diagram of the system of FIGURE 1.

Description of the preferred embodiment Considering first the general system configuration of FIGURE 1, we see that two microphones 10 and 11 are provided for receiving an incident acoustic signal at two different listening points, designated L and R. In practice, these microphones are of special design and are positioned on opposite sides of the head so as to approximate the response pattern of the left and right human ears. The output signals from these microphones are applied to first and second audio amplifiers 12 and 13, respectively, wherein they are individually amplified to a level suitable for driving output transducers 14 and 15. Transducers 14 and 15 may be compact hearing-aid type earphones suitable for insertion in the left and right human ear structures or may be contained in the hearing aid proper as in the case of eye-glass and behind-the-ear type systems.

An AGC control effect generating means 16 is coupled to the output of amplifier 12 to develop a first control effect suitable for controlling the gain of amplifier 13 inversely with variations in the output level of amplifier 12. Similarly, another control effect generating means 17 is connected to the output of amplifier 13 to develop a second control effect for controlling the gain of amplifier 12 inversely with variations in the output level of amplifier 13.

With these components alone the system may be unstable under certain conditions. To illustrate, assume that microphone 10 is ipsilateral and microphone 11 is contralateral, so that an incident acoustic signal is received at a greater level by microphone 10 than by microphone 11. Initially, the output of amplifier 12 would be greater than the output of amplifier 13 and a larger control effect would be developed by means 16 than by means 17. The control effect from means 16 would tend to reduce the gain of amplifier 13, thus further reducing the control effect produced by means 17. As the control effect from means 17 decreases, the gain of amplifier 12 increases still more, causing greater reduction in the gain of amplifier 13, still more increase in the gain of amplifier 12, and so forth. The end result would be a runaway condition in which amplifier 12 would be totally cut-off and channel 13 would operate at full gain.

To prevent this condition from occurring one aspect of the invention provides for the inclusion of across-coupling 3 means 18 to selectively combine the control effects produced by means 16 and 17. Now, a portion of the AGC control effect produced by the initially greater output signal of amplifier 12 is coupled back by means 18 and serves to oppose any further increase in the gain of amplifier 12. Likewise, a portion of the control effect developed by means 17 in response to the lower amplitude output signal of amplifier 13 is cross-coupled back by means 18 to oppose any further reduction in the gain of that channel. By selecting a proper amount of cross-coupling, the aforementioned runaway condition is prevented and the system can be made to not only preserve, but to accentuate the difference in levels between the incident signals at microphones and 11. This is desirable for the hard-of-hearing, since it gives them an increased sense of spatial separation necessary for positive orientation with respect to an incident sound.

The bilateral hearing aid system of FIGURE 2 comprises two identical audio amplifying channels 19 and 20, each of which includes a microphone, an output transducer and appropriate circuitry for amplifying the microphone output signal to a level sufficient for driving the transducer. Since the two channels are exactly identical, we will consider in detail only the circuitry of channel 19.

Channel 19 incorporates a microphone 21 having two output terminals, one of which is connected to the base of a first audio amplifier transistor 22 and the other of which is bypassed to ground by a capacitor 23. Transistor 22 is of the NPN type and is connected in a commonemitter configuration, its emitter being connected directly to ground and its collector being connected to a positive source of unidirectional potential (in this case battery 24) by a collector load resistor 25. A resistor 26 connected between the collector and base electrodes provides proper operating bias.

An interstage coupling capacitor 27 couples the amplified audio signal from the collector of transistor 22 to the base of a second audio amplifier transistor 28, which is also an NPN transistor and is connected in a connon emitter configuration. The emitter of transistor 28 is grounded and the collector is connected by a load resistor 29 to the positive terminal of battery 24. A bias resistor 30 connected between the collector and base electrodes provides proper base operating bias.

The twice-amplified signal from transistor 28 is coupled by the series combination of a gain control potentiometer 31 and a coupling capacitor 32 to the base of a third audio amplifier transistor 33. One end terminal of potentiometer 31 is conected to the collector of transistor 28, the arm is connected to capacitor 32 and the remaining end terminal is unconnected; this limits the adjustment range of potentiometer 31 so that channel 19 can never be rendered completely inoperative. Transistor 33, unlike transistors 22 and 28, is a PNP device and has its emitter terminal connected directly to the source of positive unidirectional current. The output load for transistor 33 comprises an output transducer 34, which is connected between the collector of transistor 33 and ground and is shunted by a capacitor 35 which decreases the high-fro quency response of the system for greater stability. A resistor 36 connected between the collector and base provides proper operating bias to the base. The negative terminal of battery 24, which serves as the source of unidirectional current for amplifier channel 19, is connected to ground through an on-oif switch 37 and all ground connections in channels 19 and 20 and means 38, 39 are connected to each other.

Control effect developing means 38 and 39 are connected to the outputs of amplifier channels 19 and 20, respectively, to develop AGC control voltages representative of the average output level in the respective channels. Referring to means 38, the audio output signal appearing at the collector of transistor 33 is coupled through an isolation resistor 40 to appear across the series combination of capacitor 41 and diode 42. Diode 42 conducts on positive half-cycles of the applied audio signal, and as a result a negative bias is developed across capacitor 41. To obtain a higher AGC control voltage means 38 is provided with a second diode 43 and a capacitor 44, shuntconnected across diode 42 in the familiar voltage-doubler configuration. Diode 43 conducts on negative half-cycles so that capacitor 44, which ordinarily is maintained at a slight positive bias by virtue of being connected to the biased base electrodes of the first and second audio amplifier transistors, is made less positive. In practice the maximum net voltage change is in the order of only 0.2 volt, but this is sufficient to move the quiescent operating point of the transistors closer to cut-off and bring about a substantial reduction in gain.

It will be appreciated that, although in the illustrated embodiment the channels are exactly alike, they may in practice be different and adapted to operate at different output levels, as in the instance of a person having a wide disparity in hearing impairments between his left and right ears. Furthermore, the channels may have different frequency response characteristics and different types of output transducers.

In accordance with the invention, the binaural hearing aid system of FIGURE 2 includes a novel AGC system which preserves and accentuates the differences in levels between the input signals to amplifier channels 19 and 20. In particular, the AGC control voltage from control effect generating means 39 is applied through isolation resistors 45 and 46 to the base electrodes of the first and second amplifier transistors of channel 19. The voltage thus applied works to vary the gain of these transistors by shifting their quiescent operating point to one providing a lower 18 or current gain as the audio output level from channel 20 increases. Likewise, the AGC control bias from means 38 is applied through a pair of isolation resistors '47 and 48 to the first two amplifier transistors of channel 20 and serves to vary the gain of that channel inversely with variations in the output level of channel 19. Because control effect generating means 38 and 39 do not develop a sufficient control voltage to materially affect the operation of the audio amplifier transistors at low operating levels, the AGC action is, said to be delayed. Thus, no gain adjustment takes place until the output level of the amplifying channel exceeds a certain threshold value corresponding to an above average input signal. Delayed action AGC systems of this type are common in hearing aid systems.

As was shown previously, cross-connecting the AGC system alone could result in an unstable system in which eventually one channel would operate at maximum gain and the other channel would operate at minimum gain. Therefore, and in accord with the invention, the present system achieves stability through inclusion of cross-coupling means which in its simplest and preferred form consists of a resistor 49 interconnected between the AGC control voltage outputs of control effect generating means 38 and 39. This resistor causes mixing of the two AGC control voltages, so that as the output, of one channel changes, a predetermined amount of the control voltage from the control effect developing means of that channel is coupled back to oppose further change in output. The degree of cross-coupling, which depends on the selection of resistor 49, is preselected to provide the desired transfer of bilateral level differences to the listener. It will be appreciated that other types of components, such as active sound levels is preserved. By selecting a value for resistor 49 between these extremes, a predetermined degree of enhancement of interaural level differences can be obtained within the limits of the system.

It has been established that for best spatial orientation the hard-of-hearing require greater interaural level differentials than people with normal hearing ability. Thus, it is desirable that a binaural system accentuate any interaural level difference in incident signals, and to that extent the illustrated system employs a coupling resistor of 10,000 ohms. Taking into account various factors, including the delayed action of the AGC circuits, this value provides the optimum degree of bilateral separation for persons with bilaterally impaired hearing.

By way of example, in a nominal conversation-level sound field of 75 db SPL people frequently experience interaural level differences of approximately 8 db between their ipsilateral and contralateral cars. This level difference is caused by the combined effect of the polar directional characteristics of the human ear and the head shadow effect and is sufficient to enable persons with normal hearing to quickly orientate with respect to an incident sound. However, in the case of persons with a substantial bilateral hearing impairment, say 30 db, it is necessary to amplify the 75 db SPL to approximately 105 db SPL to enable them to experience the same loudness sensation that a person with normal hearing would experience. At this level the 8 db natural incident sound differential no longer provides optimum spatial orientation, especially relative to higher level incident sounds. Accordingly, a bilateral hearing aid system constructed in accordance with the invention increases the differential for higher level incident sounds by providing less than unity cross-coupling between the cross connected AGC systems.

In the illustrated embodiment a cross-coupling impedance of 10,000 ohms provides snfficient decoupling to increase an 8 db differential to 16 db, a degree of separation which has been found to provide optimum spatial orientation to persons with a 30 db hearing loss. This increase in differential is experienced only relative to the higher level incident sounds because of the delay characteristics of the two AGC systems. For lower level incident sounds the increase is less, and for very low level sounds almost no AGC action is present and the two amplifying channels operate at approximately the same gain to substantially preserve the interaural level difference between the incident sounds.

The following are a set of component values for the circuit of FIG. 2 which have been found to provide satisfactory operation in accordance with the invention. It will be appreciated that these values are given by way of example, and that other values may be substituted therefore Without departing from the principles of the present invention.

TR22 Fairchild type SK2484 TRZS Fairchild type SK2484 TR33 Fairchild type SK3503 R26100,000 ohms R25-10,000 ohms R30100,000 ohms R293,300 ohms R31--200,000 ohms R36100,000* ohms R40--6,800 ohms R45, R4715,000 ohms R46, R48--33,000 ohms R4910,000* ohms C23-7 microfarads C270.1 microfarad C32l.0 microfarad C35.02 microfarad C410.5 microfarad C447 microfarads B24-1.55 volts silver oxide battery 6 L214,400 ohms at 1 kilohertz, 630 ohms DC L342,500 ohms at 1 kilohertz, 700 ohms DC D42, D43 Texas Instruments type T16 While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. A binaural hearing aid system comprising:

a first amplifying channel for amplifying and reproducing a first incident sound;

a second amplifying channel for amplifying and reproducing a second incident sound;

means comprising first and second automatic gain control circuits responsive to the output levels of said first and second amplifying channels for developing first and second independent control effects;

means for applying said first control effect to said second channel and said second control effect to said first channel to inversely vary the gain of each channel with variations in the output level of the other;

and cross-coupling means connected between said first and second automatic gain control circuits for causing the relative level difference of the output signals from said amplifying channels to be related in a predetermined manner to the relative level difference of said first and second incident sounds.

2. A binaural hearing aid system as described in claim 1 wherein said first and second automatic gain control means comprise circuitry for generating first and second control voltages which are amplitude-dependent on the output levels of said first and second amplifying channels respectively, and said cross-coupling means is interconnected between said control voltage generating means.

3. A binaural hearing aid system as described in claim 2 wherein said cross-coupling means comprises an impedance.

4. A binaural hearing aid system as described in claim 3 wherein said impedance is a resistor.

5. A binaural hearing aid system of the type providing separate audio output signals representative of an incident acoustic signal as received at first and second different listening points:

a first amplifying channel for developing a first amplified audio output signal representative of said incident acoustic signal as received at said first listening point;

a second amplifying channel for developing a second amplified audio output signal representative of said incident acoustic signal as received at said second listening point;

means coupled to said first and second amplifying channels for developing first and second different independent control effects representative of the average levels of said first and second amplified audio signals, respectively;

first gain control means for applying said first developed control etfect to said second amplifying channel to vary the gain thereof inversely with variations in the level of said first amplified audio signal.

second gain control means for applying said second developed control effect to said first amplifying channel to vary the gain thereof inversely with variations in the level of said second amplified audio signal;

and cross-coupling means connected between said first and second control effect developing means and said first and second amplifying channels for coupling at least a portion of said first developed control effect to said second channel and at least a portion of said second developed control effect to said first channel to cause the relative level difference between 8 the output signals of said first and secondchannel's sponsive only to theoutputlevel of saidfirst chanto be related in a predetermined manner to the relanel for developing a control effect; tive level difference of said acoustic signal as remeans for pp Said Control efiect to said nd ceived at said first and second listening points. amplifying channel to Var Y the gain thereof inversely with variations in the output level of said first am- 6. A binaural hearing aid system as described in claim 5 pllfylng channel.

5 wherein said control eifects are direct current control 23353: and said CI'OSS-COUPlll'lg means comprises an 1m- References Cited 7. A binaural hearing aid system as described in claim UNITED STATES PATENTS 6 wherein said cross-coupling impedance is a resistor. 10 3,238,304 3/ 1966 Yaita et a1. 179-1 8. A binaural hearing aid system comprising: a first amplifying channel for amplifying and repro- WILLIAM COOPER, Primary EXamiIleT ducing a first incident sound; D. W. OLMS, Assistant Examiner a second amplifying channel for amplifying and re- 15 producing asecond incident sound; US. Cl. X.R.

means comprising an automatic gain control circuit re- 1791; 330133 

